“Resistive heaters” for generating Joule heat by applying currents to thin-film resistors are widely used for various applications. Examples of such resistive heaters include micro-sized resistive heaters placed on circuit substrates or semiconductors such as silicon. A large number of attempts have been made to solve problems due to the size of the micro-sized resistive heaters. See, for example, Japanese Unexamined Patent Application Publication Nos. 58-134764, 3-164270, and 61-219666 and Japanese Patent No. 2811209. Techniques relating to these resistive heaters are usually used to heat specific micro-regions (several micrometers square) or relatively large-area regions which are several millimeters to several centimeters square such that semiconductor devices mounted on such regions are heated.
In a case where a square region or a rectangular region which has a small aspect ratio and which is therefore close to a square shape is heated, the shape of a resistive heater placed in the region is not particularly limited. Therefore, a desired object can be readily achieved by allowing the resistive heater to have such a shape that the temperature distribution in the region can be desirably adjusted. For the electrical resistance of resistive heaters, since a large number of holes can be bored in a sheet-shaped resistive heater, the electrical resistance of the heater can be readily adjusted by varying the size and/or number of the holes as disclosed in Japanese Unexamined Patent Application Publication No. 58-134764.
For the resistive heater for heating the square or rectangular region, the temperature distribution obtained by the resistive heater and the electrical resistance of the resistive heater can be adjusted by varying the shape of the resistive heater.
The electrical resistance of the resistive heater is a key factor to determine the necessary performance, for example, the maximum voltage, of an external circuit for driving the resistive heater. If the resistive heater has a large electrical resistance, an extremely high voltage must be applied to the driving circuit. In consideration of the voltage (about 5 to 12 V) of a power supply, connected to a control circuit (usually including semiconductor devices), for controlling the temperature, there is a problem in that these circuits cannot be connected to a common power supply. Thus, it is necessary to adjust the electrical resistance of the resistive heater.
On the other hand, an optical component, for example, “a thermooptic phase shifter”, used for optical communication includes a resistive heater (see, for example, Japanese Unexamined Patent Application Publication No. 6-34926). This resistive heater includes a resistor having a width of several micrometers to several tens micrometers and a length of about 2 to 5 mm. The length of the resistor is extremely greater than the width thereof. Therefore, this resistive heater is different from that resistive heater in that the resistor has a narrow line shape (a narrow stripe shape). The thermooptic phase shifter includes an optical waveguide section having a width of about 5 μm and a length of about 2 to 5 mm. In order to selectively heat the optical waveguide section having such a shape using this resistive heater, the resistor must also have a narrow line shape.
Since the resistor has a width of several micrometers, it is difficult to arbitrarily adjust the electrical resistance of the resistor by varying the shape thereof in the same manner as that described above. This is because a micromachining technique is necessary to shape the resistor.
The resistor is allowed to have a thickness of up to several hundreds nanometers because of the reason due to a semiconductor process used to form the thermooptic phase shifter. That is, the thickness of the resistor is limited. The number of materials for forming the optical waveguide section is not very large because such materials must have good machinability, high stability, and high adhesion to a glass material for forming the optical waveguide section.
As described above, in the resistive heater included in the optical component, there is a limitation that the resistor must have a narrow line shape; hence, it is very difficult to prepare a heating element (in particular, a heating element with low electrical resistance) with desired electrical resistance properties by improving the shape of the resistor. Furthermore, it is not easy to adjust the thickness of the resistive heater or change a material for forming the resistive heater as required because of process and material constraints.
There are known techniques relating to the present invention as described below.
Japanese Unexamined Patent Application Publication No. 2001-301219 discloses a thermal print head including a wire resistor. The thermal print head, as specified in claim 1 of this patent document, includes “a linear resistor, a power supply line, a grounding line, and an integrated circuit device, wherein the integrated circuit device includes a plurality of transistors each including respective first electrodes connected to the power supply and respective second electrodes connected to the grounding line and also includes a plurality of pads for connecting the second electrodes to the grounding line and the resistor generates heat when a current is applied to the resistor by bring the transistors into conduction”. According to such a configuration, the following advantages can be achieved: “the second electrodes can be connected to the pads with short wires, the wires therefore have low resistance, a difference in wiring resistance between the transistors is small, electricity consumption is low, the life of a battery included in the thermal print head is long if the thermal print head is of a portable type, the thermal print head can be driven with a low-voltage battery because a voltage drop due to the wiring resistance is small, the quality of an image formed by the thermal print head is high because a difference in wiring resistance between the transistors is small and because a difference in temperature between portions of the resistor is small”.
In the thermal print head disclosed in Japanese Unexamined Patent Application Publication No. 2001-301219, the resistor and the power supply line are connected to each other with a plurality of spaced wires and the first electrodes of the transistors are connected to the resistor with wires. The first and second electrodes of the transistors correspond to the drains and sources of MOS transistors, respectively. If one of the transistors in the integrated circuit device is turned on, a current flows from the power supply line to the grounding line through the resistor and the transistor. Since the current flows in two wires for connecting the power supply line to the resistor and flows in a portion of the resistor that is sandwiched between the two wires, the resistor portion can be selectively heated.
Japanese Unexamined Patent Application Publication No. 2002-008901 discloses a thin-film resistor, a hybrid IC, and a microwave monolithic integrated circuit (MMIC). In the thin-film resistor, “a first electrode and second electrode connected to thin-film resistor portions have narrow, irregular sections extending in the direction that the first and second electrodes face to each other; sides of the irregular sections of the first and second electrodes are arranged at predetermined intervals; and the thin-film resistor portions are arranged between the sides facing to each other”. That is, in the thin-film resistor, an end section of the first electrode is shaped so as to have an interdigital shape so that the irregular electrode sections are formed, an end section of the second electrode is shaped so as to have an interdigital shape so that the irregular electrode sections are formed, and the electrode sections are engaged with each other in such a manner that the interdigital electrode sections of the second electrode are placed in spaces between the interdigital electrode sections of the first electrode. The thin-film resistor portions are separately placed in spaces between the interdigital electrode sections engaged with each other.
According to such a configuration, the following advantages can be achieved: “the thin-film resistor can be shaped so as to have a size close to the width of wires and a region for forming the thin-film resistor can therefore be formed so as to have a desired characteristic impedance”.
If the operational stability and reliability of resistive heaters are regarded as most important, tantalum nitride (TaN) is usually used to prepare resistors. Thin-film heaters, made of TaN, for semiconductor circuits have a large electrical resistance because the resistivity of TaN is usually high, 200 to 300 μΩ·cm, under conditions for stably forming layers. If, for example, a TaN layer is processed into fine wires having a thickness of 200 nm a width of 10 μm, and a length of 2 mm, the wires have an electrical resistance of 2 to 3 kΩ. In order to allow a wire resistor, made of TaN, having such an electrical resistance to generate 300 mW of heat, the voltage necessary to energize the wire resistor is very high, 17 to 30 V.
An attempt to prepare a small-sized, precisely controllable heating element including a TaN wire resistor causes a problem, i.e., an increase in the size of a driving power supply. Hence, the attempt is impossible. This can be applied to titanium nitride (TiN), as well as TaN, having a relatively large resistivity.